(1) Field of the Invention
The present invention relates to a radio communication system, radio communication method, radio transmitter and radio receiver, and more particularly to a technique suitable for use in a radio communication system employing a plurality of antennas (multiantenna).
(2) Description of the Related Art
[Radio Transmission using Multiantenna]
In recent years, a requirement has been made for a mobile communication system capable of realizing low-SNR (Signal to Noise Ratio) operations and high-speed data transmissions, and such a mobile communication system has been under examination. In this case, as multiantenna systems, for example, there have been known MIMO (Multi-InputMulti-Output) and AAA (Adaptive Array Antenna).
The former (MIMO) is a technique employing a transmitter having a plurality of antennas and a receiver having a plurality of antennas to carry out the transmission and reception of independent data stream through each of the antennas for improving the transmission rate, while the latter (AAA) is a technique of carrying out the transmission and reception of one data stream after the formation of beams with directivity through the use of a transmitter having a plurality of antennas and a receiver having a plurality of antennas for improving communication quality and achieving interference suppression.
The research and development of a multiantenna system for use in a mobile communication system and the partial introduction thereinto prevail at present. For example, the AAA was introduced into a portion of a PHS (Personal Handyphone System) and a report was made as “offer of more stable call environment, follow-up of moving telephone, improvement of effective frequency utilization factor to twice or more” (for example, see non-patent document 1 mentioned later). Moreover, a product made by applying the MIMO technique to a radio LAN system appeared, and a report was made as “realization of twice as high as conventional high-speed radio throughput” (for example, see non-patent document 2 mentioned later).
Thus, the application of the multiantenna technique to a mobile communication system already started.
Secondly, a description will be given hereinbelow of a (systematic) research situation of a multiantenna transceiving technique for the application to the mobile communication system in the future.
As various types of multiantenna transceiving techniques, there are (A) MIMO multiplexing method, (B) MIMO diversity method and (C) AAA transceiving method, and a study has been made thereon (for example, see non-patent document 3 mentioned later). A comparative table mentioned in this document is the following table 1.
TABLE 1Comparison Among MultiantennaTransmitter-Receiver Configurations(A) MIMO(C) AAAMultiplexing(B) MIMOTransmissionMethodDiversity MethodMethodEffectIncrease inIncrease inIncrease inInformationDiversity GainAverage ReceptionBit Rate(Improvement ofSIR(High-SpeedCommunication(Improvement ofData Transmission)Quality)Communication Quality)Interval ofLargeLargeSmallAntennas RequiredFadingHighSignalTransmissionAntenna Gain:CorrelationDemultiplexingDiversity Gain:HighCapability:LowLowLowSignalTransmissionAntenna Gain: LowDemultiplexingDiversity Gain:Capability:HighHighInterferenceNoNoExistSuppressionEffect between OwnCell and Other CellRF CircuitUnnecessaryUnnecessaryNecessaryCalibration
As seen from this table 1, the MIMO techniques (A) and (B) are advantageous when the inter-antenna correlation is low (close to no correlation), while the AAA technique (C) is advantageous when the inter-antenna correlation is high (close to 1). Moreover, in the case of the MIMO multiplexing method (A), the information rate is increasable by increasing the number of antennas, while in the case of the MIMO diversity method (B) and the AAA transmission method (C), although the information rate does not increase with an increase in the number of transmission antennas, the diversity gain and the antenna gain are improvable.
In connection with the results in this non-patent document 3, the following patent document 1 (for example, Claims 1 to 3) discloses a method of, in a case in which an SIR (power ratio between a desired signal and an adjacency interference) is below a given threshold, making the switching from a plurality of data streams independent of each other to a plurality of data streams produced from the same data stream. In this case, it can be considered that the former (the plurality of streams) corresponds to the MIMO multiplexing method (A) in the foregoing table 1, while the latter (the same stream) corresponds to the MIMO diversity method (B).
In addition, the following patent document 2 (for example, Claims 1 to 4, 7) discloses a signal transmission system of carrying out a transmission mode for transmitting signals independent of each other from a transmission antenna and a mode for signals dependent on each other through the use of means for estimating a propagation line between transmission and reception, means for estimating a communication capacity at every transmission mode, means for determining a transmission mode and means for calculating a spatial correlation value between antennas in the reception side. With respect to these transmission modes, in the foregoing table 1, the former (independent signal) corresponds to the MIMO multiplexing method (A), while the latter (dependent signal) corresponds to a technique close to the AAA transmission method (C).
Patent Document 1: Japanese Patent Laid-Open No. 2004-72624
Patent Document 2: Japanese Patent Laid-Open No. 2004-194262
Non-Patent Document 1: WILLCOMinc., “H” • Anshinda Phone Adaptive Array • Antenna”, [online], [Retrieval Date: Jun. 15, 2005], Internet <URL:http://WWW.willcom−inc.com//p_s/products/h/antenna.h tml>
Non-Patent Document 2: Buffalo Co., Ltd., “AdvancedRadio LAN Technology-MIMO-Mounted” Radio Broadband Router”, [online], [Retrieval Date: Jun. 15, 2005], Internet <URL:http://buffalo.melcoinc.co.jp/products/catalog/network/wzr−g108/index.html>
Non-Patent Document 3: Mamoru SAWABASHI and 5 persons, “Outline of Broadband Radio Access System” NTT Technology Journal (the July 2000 issue), p. 20-22
In a radio communication apparatus having a multiantenna, it is considered that a higher data transmission is made in accordance with a situation of communication at any given time for improving the sector (cell) throughput. For realizing this, both the MIMO and AAA may be provided to be selectively used in a switching fashion. Accordingly, it is an object to provide a method of combining the MIMO and the AAA in cooperation.
In the case of a radio line, for compensating for the phase turn occurring in a propagation line, a pilot signal is used which serves as a phase reference. According to the MIMO technique, an orthogonal pilot for each antenna is used to estimate a channel for a signal to be transmitted from each antenna. This pilot signal is used in common among all the users and, hence, it is referred to as a common pilot.
When the MIMO pilot signal is used for the AAA, since a data unit produces and transmits a beam with directivity, different propagation paths are employed between the pilot signal and the data signal and, hence, difficulty is encountered in employ it for the channel estimation. For this reason, there is a need to add a pilot signal with a different directivity and transmit it.
However, this leads to a decrease in transmission rate (throughput) and requires the information on the added pilot signal and the processing therefor. The pilot signal for the AAA is referred to as an individual pilot because it is required for each user.